1. Field of the Invention
This invention relates to the manufacture of polyisocyanurate foams from a foam-forming composition containing a polyester polyol and a blowing agent comprising (a) chlorodifluoromethane (HCFC-22) or (b) a mixture of HCFC-22 with at least one other blowing agent.
2. Description of the Prior Art
It is well known in the manufacture of rigid polyurethane and polyisocyanurate foams to employ fluorocarbons, such as trichlorofluoromethane, as the blowing agent. These fluorocarbon compounds boil or exhibit a significant vapor pressure at ambient temperatures and are volatilized during the exothermic reaction of an isocyanate with an active hydrogen-containing compound, such as a polyol. The expanding gas is entrapped within the reaction mixture and forms a cellular structure. While the foam industry has had good results using the conventional fluorocarbon blowing agents, the agents have come under attack in recent years on the ground that they give rise to environmental problems concerned with ozone depletion in the stratosphere. Accordingly, the search is ongoing for alternative blowing agents which are less hazardous to the environment.
The use of polyester polyols (e.g., aromatic polyester polyols) as low cost polyol extenders in the production of the rigid polyurethane and polyisocyanurate foams is also well known. Since these polyols can contribute unique properties to the foam, such as low flammability and low smoke, it is desirable to maximize their content in the polyol component of a foam-forming composition. Unfortunately, a common problem with these polyester polyols is that they characteristically are poorly compatible with the conventional fluorocarbon blowing agents.
Various compatibilizing co-polyols have been developed to overcome this incompatibility problem and achieve higher levels of polyester polyols in the polyol component. A particularly advantageous approach, which also involves use of an alternative blowing agent characterized by a low ozone depletion potential, is provided by U.S. Pat. No. 4,636,529, wherein chlorodifluoromethane is substituted for a portion of the conventionally used dichlorodifluoromethane and trichlorofluoromethane blowing agents. The patent discloses that this blowing agent substitution permits the proportion of the aromatic polyester polyol in the polyol component of the foam system to be increased and exemplifies the production of foams wherein the aromatic polyester polyol constitutes 50 weight % of this component. The foam system is doubly benefitted due to the increased amount of aromatic polyester polyol and the inclusion of the chlorodifluoromethane, a hydrogen-containing chlorofluorocarbon, whose relatively short lifetime in the atmosphere presents a reduced risk of ozone depletion.
Despite such advances in the art, there is still a need for improved foam systems which provide increased use of both polyester polyols and more acceptable fluorocarbon substitutes.